This invention relates to a liquid crystal display and, more particularly, to a liquid crystal display equipped with an optical guide plate and a process for fabrication thereof.
An optical guide plate is incorporated in a liquid crystal display together with a light source such as a lighting equipment or a back light. The light source radiates light, and the optical guide plate directs the light toward an array of pixels. The optical guide plate makes the light source serve as a surface illuminant.
A typical example of the optical guide plate has a diffusing surface, a light output surface and an incident surface. The diffusing surface is opposed to the light output surface, and the incident surface extends between the diffusing surface and the light output surface. The light source is attached to the incident surface, and the light is incident on the incident surface. Most of the light repeats the reflection at the boundary between the optical guide plate and the air due to the difference of optical density therebetween, and proceeds in the optical guide plate. While the light is proceeding, the light is irregularly reflected on an irregular reflecting means. When a light component is incident on the irregular reflecting means at the incident angle equal to or less than the critical angle, the light component is output from the light output surface. The irregular reflecting means is implemented by a white printed layer of diffusing material such as white ink. Otherwise, the diffusing surface is granulated or roughened so as to make it granulated.
User requests the manufacturer to make the surface illuminant thin. The granulated surface is suitable for the user""s request. An example of the prior art optical guide plate with the granulated surface is disclosed in Japanese Patent Publication of Unexamined Application (laid-open) No. 2-176629.
FIG. 1 illustrates the prior art optical guide plate disclosed in the Japanese Patent Publication of Unexamined Application. The prior art optical guide plate comprises an optical guide body, a line light source 3, a curved mirror 4 and a reflecting sheet 5. The optical guide body 1 has a diffusing surface 2a, a light output surface 2b and a side surface therebetween. The diffusing surface 2a is inclined with respect to the light output surface 2b, and the distance therebetween is decreased from the side surface toward the other side surface. The side surface serves as an incident surface, and the line light source 3 is opposed to the side surface. The curved mirror 5 is attached to the optical guide body 1 so as to encapsulate the line light source 3 inside the space defined by the curved mirror and the side surface. The diffusing surface 2a is granulated, and the reflecting sheet 5 is attached to the granulated diffusion surface 2a. The air is confined between the granulated diffusion surface 2a and the reflecting sheet 5.
When the line light source 3 is energized, the line light source 3 radiates light, and the light is incident onto the side surface. The incident light proceeds in the prior art optical guide plate. The light radiated from the optical guide body 1 toward the reflecting sheet 5, and is reflected thereon. The light is partially reflected on the diffusion surface 2a. Thus, the light repeats the reflection and refraction at the boundary between the air and the optical guide body 1, and proceeds toward the side surface opposite to the incident surface. The light is partially reflected on the diffusing surface 2a, and is radiated from the light output surface 2b. 
A problem is encountered in the prior art optical guide plate in that the luminance on the light output surface 2b is gradually reduced toward the side surface opposite to the incident surface. Another problem is that the light output surface 2b is not uniform in brightness.
It is therefore an important object of the present invention to provide an optical guide plate, which has a light output surface constant in the luminance over the distance between an incident surface and the opposite surface and, accordingly, uniform in the brightness thereover.
It is also an important object of the present invention to provide a process for fabricating the optical guide plate.
The present inventor contemplated the problem inherent in the prior art optical guide plate, and found that the air was locally evacuated from between the optical guide body 1 and the reflecting sheet 5. The non-uniform brightness and the reduction of luminance were derived from the local evacuation of the air as follows. The surface roughness of the diffusion surface 2a was 1 to 5 microns, and the optical guide body 1 tended to locally hold the reflecting sheet 5 in contact with the diffusion surface 2a. The air was evacuated from between the part of the diffusion surface 2a and the reflecting sheet 5. The total reflection took place on the part of the diffusion surface 2a, and the part of the diffusion surface interrupted the optical propagation through the air. If there was the air constant over the boundary between the diffusion surface 2a and the reflecting sheet 5, all the light was not taken into the optical guide body 1, again, due to the difference in density between the optical guide body 1 and the air. Thus, the amount of light was reduced in the direction leaving the incident surface, and the luminance was reduced due to the shortage of the light. On the other hand, the total reflection locally increased the amount of output light, and made the brightness on the light output surface 2b non-uniform.
To accomplish the object, the present invention proposes to form micro-projections large enough to space the reflecting sheet from the diffusion surface.
In accordance with one aspect of the present invention, there is provided an optical guide plate used in a liquid crystal display comprising an optical guide body formed of a transparent material, and including an incident surface, a granulated diffusion surface for scattering light components, a light output surface for radiating the light components therefrom and micro-projections formed on the granulated diffusion surface and having peaks spaced from the granulated diffusion surface wider than the granulation, a reflecting sheet supported by the peaks of the micro-projections so as to form an air layer therebetween, and a light source opposed to the incident surface for supplying the light components to the optical guide body.
In accordance with another aspect of the present invention, there is provided a process for fabricating an optical guide plate, comprising the steps of forming a transparent optical guide body having an incident surface, a granulated diffusion surface, a light output surface and micro-projections formed on the granulated diffusion surface and having peaks spaced from the granulated diffusion surface wider than the granulation, and assembling a reflecting sheet and a light source with the transparent optical guide in such a manner that the reflecting sheet is supported by the peaks and that the light source is opposed to the incident surface.